Early to mid-Holocene human activity exerted gradual influences on Amazonian forest vegetation.

Humans have been present in Amazonia throughout the Holocene, with the earliest archaeological sites dating to 12 000 years ago. The earliest inhabitants began managing landscapes through fire and plant domestication, but the total extent of vegetation modification remains relatively unknown. Here, we compile palaeoecological records from lake sediments containing charcoal and from pollen analyses to understand how human land-use affected vegetation during the early to mid-Holocene, and place our results in the context of previous archaeological work. We identified gradual, rather than abrupt changes in forest openness, disturbance and enrichment, with useful species at almost all sites. Early human occupations occurred in peripheral sites of Amazonia, where natural fires are part of the vegetation dynamics, so human-made fires did not exert a novel form of disturbance. Synchronicity between evidence of the onset of human occupation in lake records and archaeological sites was found for eastern Amazonia. For southwestern and western Amazonia and the Guiana Shield, the timing of the onset of human occupation differed by thousands of years between lake records and archaeological sites. Plant cultivation showed a different spatio-temporal pattern, appearing ca 2000 years earlier in western Amazonia than in other regions. Our findings highlight the spatial-temporal heterogeneity of Amazonia and indicate that the region cannot be treated as one entity when assessing ecological or cultural history. This article is part of the theme issue 'Tropical forests in the deep human past'.

Human practices behind the aquatic and terrestrial ecological decoupling to climate change in the tropical Andes.

Anthropogenic climate change and landscape alteration are two of the most important threats to the terrestrial and aquatic ecosystems of the tropical Americas, thus jeopardizing water and soil resources for millions of people in the Andean nations. Understanding how aquatic ecosystems will respond to anthropogenic stressors and accelerated warming requires shifting from short-term and static to long-term, dynamic characterizations of human-terrestrial-aquatic relationships. Here we use sediment records from Lake Llaviucu, a tropical mountain Andean lake long accessed by Indigenous and post-European societies, and hypothesize that under natural historical conditions (i.e., low human pressure) vegetation and aquatic ecosystems' responses to change are coupled through indirect climate influences-that is, past climate-driven vegetation changes dictated limnological trajectories. We used a multi-proxy paleoecological approach including drivers of terrestrial vegetation change (pollen), soil erosion (Titanium), human activity (agropastoralism indicators), and aquatic responses (diatoms) to estimate assemblage-wide rates of change and model their synchronous and asynchronous (lagged) relationships using Generalized Additive Models. Assemblage-wide rate of change results showed that between ca. 3000 and 400 calibrated years before present (cal years BP) terrestrial vegetation, agropastoralism and diatoms fluctuated along their mean regimes of rate of change without consistent periods of synchronous rapid change. In contrast, positive lagged relationships (i.e., asynchrony) between climate-driven terrestrial pollen changes and diatom responses (i.e., asynchrony) were in operation until ca. 750 cal years BP. Thereafter, positive lagged relationships between agropastoralism and diatom rates of changes dictated the lake trajectory, reflecting the primary control of human practices over the aquatic ecosystem prior European occupation. We interpret that shifts in Indigenous practices (e.g., valley terracing) curtailed nutrient inputs into the lake decoupling the links between climate-driven vegetation changes and the aquatic community. Our results demonstrate how rates of change of anthropogenic and climatic influences can guide dynamic ecological baselines for managing water ecosystem services in the Andes.